Astrophysics Highlights from the APS April Meeting

Feeding Time for Milky Way’s Black Hole
Astronomers will get the extremely rare opportunity to observe a black hole eating a light snack. A gas cloud three times the mass of Earth is about to skim the outer rim of Sagittarius A*(Sgr*) — the supermassive black hole at the center of our galaxy — and some of the dust is expected to get pulled into the black hole’s gaping maw. Over a dozen telescopes will observe various stages of the event. The closest approach of the gas cloud is beginning now and will last about one year, with matter expected to continue to fall in for a few decades.

At the APS April Meeting, Daryl Haggard of Northwestern University spoke to reporters about what this event reveals about the feeding habits and growth patterns of black holes. Do black holes grow rapidly at the beginning of their lives and then plateau, similar to humans? Or do the black holes eat mid-sized meals periodically, causing them to grow in fits and starts? The encounter will provide new information about how frequently these events may occur. There is also the hope that the in-falling gas will cause the accretion disk of Sgr* to light up, which could provide some interesting comparison data for observations of accretion disks around other black holes.

Stefan Gillessen of the Max Planck Institute for Extraterrestrial Physics says the movement of the gas cloud will allow astronomers to probe the atmosphere around Sgr* and finally obtain observational evidence to compare to simulation. The cloud has already been stretched out by the pull of the black hole, and some of the gas may have already swung around to the other side. Predictions suggest the pull of the black hole will undo the ball-shape of the dust cloud and send it swirling, like milk poured into stirred coffee. But how will those predictions compare with the real thing? “All I can say,” concluded Gillesen, “Is that we’d better watch.”

IceCube and Neutrino Astronomy
Last November, the IceCube collaboration published results confirming their detection of high-energy neutrinos originating outside the solar system. At a press conference at the APS April Meeting, Christopher Weaver, a graduate student at the University of Wisconsin, spoke about a new analysis that confirmed the November announcement. Via a different type of analysis, Weaver measured the same rate of arrival of high-energy astrophysical neutrinos to the IceCube detector.

Weaver’s analysis, as yet unpublished, looks specifically for muon neutrinos, which create very clear, pointed tracks through the IceCube detector. These tracks make the muon neutrinos easily distinguishable from background, and their orientation may also help scientists trace the neutrinos back to their sources. The analysis also looked at a wider energy range than the November starting analysis, providing researchers with a better understanding of the background neutrinos reaching the detector.

The analysis also identified two new, very-high-energy neutrinos close to the PeV range. “The case remains the same, and in fact we have a stronger case now,” said Naoko Neilson, a postdoc at the University of Wisconsin who delivered a plenary session talk. She defended the reasons for pursuing neutrino astronomy, and compared it to observations in different photon wavelengths: Optical, radio, infrared and other wavelengths each reveal new information about a single astrophysical source.

To complement these techniques, IceCube will have to be able to pinpoint the sources of cosmic neutrinos. Currently, the cosmic neutrinos detected by IceCube appear as a haze in the sky, and Neilson says they could be coming from as many as 50 separate sources. But with more time and more data, Neilson says she’s confident the experiment will reach this goal.

“Think about gamma-ray astronomy: We take for granted that it is astronomy now,” said Neilsen. “It all started with diffuse celestial radiation, and then in the '70s and '80s people started resolving gamma-ray sources and now we have a very thick catalogue [of gamma-ray sources]…. Hopefully in the near future, I can come back to you and say we’ve measured the first handful of neutrino sources. And hopefully before my career or my life is done, we can get to the point where we talk about a neutrino astronomy catalogue.”

HAWC Observatory Online
At a press conference, Petra Huentemeyer of Michigan Technological University gave a status update and early results from the High-Altitude Water Chernkov (HAWC) observatory. HAWC will produce a wide-field picture of the universe in TeV gamma rays and cosmic rays. With just one third of its total planned array online, HAWC has already exceeded the sensitivity of its predecessor MILAGRO.

In recent years, the Fermi Gamma-Ray Space Telescope, which detects photons with energies up to 300 GeV, has provided a tremendous wide-field map of the gamma-ray universe, and identified hundreds of point sources that have been studied in detail by non-survey telescopes. If HAWC reaches its full capability, it will provide a similar all-sky gamma-ray map up to 100 TeV. In the same fashion as Fermi, Huentemeyer says HAWC will work cooperatively with TeV point-source telescopes like VERITAS, HESS and MAGIC.

HAWC’s sky map thus far has succeeded in identifying gamma- ray standard candles such as the Crab Nebula. In addition, it shows two mysterious cosmic ray excesses originally spotted by MILAGRO, and a new, third excess. Looking forward, Huentemeyer says HAWC may have a better chance of figuring out what those excesses are–attempts at explanation range from magnetic-field turbulences to dark matter. But HAWC will also try to help answer major questions about how cosmic rays are produced, how they are accelerated and where they come from.

“It has a high discovery potential,” said Huentemeyer. “This is just the beginning.”